Vapor phase systems are known in which a processing vapor is provided in a vessel and into which a product is introduced to accomplish a particular process or operation. One such system is known for vapor phase soldering wherein solder on a workpiece is caused to melt or reflow by the heat from a condensing heated vapor. In such a vapor phase soldering system, a fluorocarbon or other suitable liquid is provided in a tank and is heated to a temperature sufficient to produce a hot saturated vapor above the surface of the liquid and in equilibrium therewith. This liquid preferably is a nonconducting chemically stable inert liquid having an atmospheric boiling point slightly above the soldering temperature and which provides non-oxidizing, nonflammable vapor. The tank is usually open to the atmosphere to facilitate product entry and removal, and the vapor can emanate from the open tank into the atmosphere. The release of process vapor into the atmosphere can present a health risk, and the presence and magnitude of such emissions are becoming the subject of increasingly stringent government and industry health and safety standards. Moreover, the liquid providing the vapor can be relatively expensive, and thus for economy of system operation, loss of the liquid through vapor emission into the atmosphere should also be minimized.
One technique for minimizing vapor loss is shown in U.S. Pat. No. 3,904,102 wherein a secondary vapor blanket is provided over the primary or processing vapor to shield the primary vapor from the atmosphere. The secondary vapor blanket is most typically formed of Freon TF (R-113) which stratifies between the primary vapor and the atmosphere by virtue of a lower boiling point and a lower density than the primary vapor. Such a dual vapor system presents several disadvantages. The system can be more complex than single vapor systems by reason of the additional cooling and recovery equipment required for the secondary fluid. The secondary vapor is not in contact with its own boiling phase but rather is exposed to the higher temperature primary vapor; thus, the secondary vapor is caused to exist at a superheated temperature, usually about 180.degree.-225.degree. F., which causes breakdown of the material into components which can be toxic and corrosive. In the case of Freon TF, gases can form and combine with water to produce hydrochloric and hydrofluoric acids, which can affect the processing system and add to the cost of its maintenance. Such acid contaminants exist in the secondary vapor to some degree even when employing acid removal procedures on the condensed secondary liquid. Moreover, the boiling phase of the primary liquid can generate pollutants such as perfluoroisobutylene (PFIB) which can enter the atmosphere.
Systems have been proposed using tank covers to the processing vapor, however, vapor loss still occurs when the cover is opened to admit product entry and removal. An improved system for minimizing the problems of vapor loss is shown in U.S. Pat. No. 4,077,467 of the same inventor and assignee as herein. In the system therein described, the primary and secondary liquids are separated and contained within closed individually controlled vapor-lock chambers. The separate chambers minimize vapor communication therebetween, inhibit the flow of contaminants, minimize vapor loss, and avoid fluid breakdown due to excessive heating of the secondary liquid. The separation of the chambers is achieved by movable doors which sequentially open to admit the work and close behind the work; after the soldering is completed, the doors operate in reverse sequence.
Another system is shown in U.S. Pat. No. 3,375,177, wherein a convection flow is created by a first set of cooling coils, which flow is generally upward in the center of the vessel and downward along the vessel walls. A second set of cooling coils is positioned about the vessel walls and spaced above the first set of coils and which functions to cause a downward convection flow at the vessel walls coincident with the vapor flow from the first set of cooling coils. The vapor rising upward in the vessel is entrained and drawn around the second coils which function to condense the vapor to cause a convection current to be drawn toward the second coils and downward along the tank walls. However, the upward vapor flow in the central portion of the vessel can still result in substantial vapor loss through the vessel opening.